Abstract
Aggregation and self-assembly are influenced by molecular interactions. With precise control of molecular interactions, in this study, a wide range of nanostructures ranging from zero-dimensional nanospheres to hierarchical nanoplates and spindles have been successfully synthesized at ambient temperature in aqueous solution. The nanostructures reported here are formed by aggregation of spherical seed particles (monomers) in presence of quaternary ammonium salts. Hydroxide ions and a magnetic moment of the monomers are essential to induce shape anisotropy in the nanostructures. The cobalt nanoplates are studied in detail, and a growth mechanism based on collision, aggregation, and crystal consolidation is proposed based on a electron microscopy studies. The growth mechanism is generalized for rods, spindles, and nearly spherical nanostructures, obtained by varying the cation group in the quaternary ammonium hydroxides. Electron diffraction shows different predominant lattice planes on the edge and on the surface of a nanoplate. The study explains, hereto unaddressed, the temporal evolution of complex magnetic nanostructures. These ferromagnetic nanostructures represent an interesting combination of shape anisotropy and magnetic characteristics.
Highlights
The synthesis of ferromagnetic nanomaterials with complex functional architectures has seen rapid development during the past decade [1,2,3]
The present study contributes in several new ways. (i) We propose a new method to form hierarchical nanostructures of cobalt by an aqueous route under ambient conditions, using quaternary ammonium salts; (ii) a variety of shapes, from zero-dimensional spheres to hierarchical interlocked nanoplates and spindles, is obtained through small variations of the protocol; (iii) nanostructures are obtained only by the precise control of the synthesis parameters without the addition of surfactants or applying external magnetic fields; (iv) we use electron microscopy to capture important modes of aggregation and growth for highly ordered hierarchical nanostructures; (v) we identify the importance of crystal twinning and various forces necessary for the formation of anisotropies
During the formation of nanoplates four important observations were made: (i) Nanoplates are formed only at a certain concentration of hydroxide ions; (ii) after replacing the hydroxide anion in tetraalkylammonium hydroxide with other ions nanoplates do not form; (iii) nanoplates are formed only under ambient growth conditions, at higher temperatures rods are obtained; (iv) steric hindrance by the alkyl group of the ammonium salt changes the shape of the nanostructures
Summary
The synthesis of ferromagnetic nanomaterials with complex functional architectures has seen rapid development during the past decade [1,2,3]. We report the formation of various nanostructures of cobalt such as nanoplates, nanorods, nanospheres and nanospindles with well-defined crystal planes by performing an aqueous syntheses at ambient temperature.
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